1. Field of the Invention
The present invention relates to serial electrophotographic apparatuses, and more particularly to a serial electrophotographic apparatus in which a latent image is formed on a recording drum, and toner is used to print a visible image on recording paper.
A demand for inexpensive and compact electrophotographic apparatuses has resulted in the development of serial electrophotographic printers. In electrophotographic printers, a carriage for carrying a printhead is capable of performing an electrophotographic process. In these electrophotographic printers, recording paper is transported in a direction perpendicular to a direction in which the carriage is translated so that a transferring unit effects an image transfer onto the recording paper. A roller shaped fixing unit disposed ahead of the carriage in the direction of transportation effects fixing. In such a printer, safety measures must be taken against the heat generated within.
2. Description of the Prior Art
FIGS. 1A and 1B show a construction of a conventional serial electrophotographic printer 11, FIG. 1A being a partial top view of the printer, and FIG. 1B being a cross-sectional view of a carriage.
The serial electrophotographic printer 11 shown in FIGS. 1A and 1B is disclosed in Japanese Laid-Open Patent Application No. 61-152463. In the electrophotographic printer 11, a shaft 14 is disposed parallel with rollers 13a and 13b for transporting recording paper 12. A carriage 15 is driven by a motor (not shown) and guided by the shaft 14 to be movable in a direction perpendicular to a direction in which the recording paper is transported. A fixing unit 16 longer than the width of the recording paper 12 is fixed ahead of the carriage 15 in the direction in which the recording paper is transported. The fixing unit 16 may be equipped with a halogen lamp as a heat source. A transferring unit 17 is disposed below the recording paper 12 to lie in the direction in which the carriage 15 is transported.
The carriage 15 carries an image carrying body (recording drum) 21 which is rotated at a peripheral speed synchronized with the movement of the carriage 15. The surface of the image carrying body 21 is uniformly charged by a charger 22 (a charging roller) 22, and an electrostatic latent image is formed by an exposer 23. The electrostatic latent image is made visible to become a toner image by a developing roller 26 which is formed adjacent to the image carrying body 21 and supplies toner 25 stored in a developing unit 24 to the image carrying body 21. Toner image formed on the image carrying body 21 is transferred to the recording paper 12 by the transferring unit 17 disposed opposite to the image carrying body 21, the recording paper 12 being led through a space between the image carrying body 21 and the transferring unit 17. The recording paper 12 is transported so that a part which has undergone a transferring process faces the fixing unit 16, whereupon the image is fixed.
Excessive increase in the temperature of the fixing unit 16 is usually prevented by disposing a temperature fuse or the like in the neighborhood of the heat source, and by stopping the power supply in the event of an excessive increase in the temperature.
The applicants proposed a serial electrophotographic printer in which the fixing unit is mounted in the carriage, and fixing is done by rotating a fixing roller in cooperation with the recording drum, and in which an induction heating coil provided separately is used as a means to heat the fixing roller in a non-contact manner (Japanese Patent Application No.5-217609).
Induction heating is a method whereby magnetic flux generated by the induction coil is applied to the fixing roller so that an eddy current generated within the fixing roller and an electric resistance of the fixing roller interact to produce heat. Since the coil is a conductive body, an eddy current is generated in the coil itself, resulting in a disadvantage that the induction coil itself is heated due to Joule heat generated by a high-frequency current that flows in the coil and the eddy current.
The high-frequency current supplied to the coil is usually large and the magnetic flux density in the induction coil disposed in a narrow space is high. Thus, the eddy current generated in the coil is large (heat becomes more intense toward the center of the coil, thus producing greater heat therein than at the ends). Hence, the heat produced in the induction coil itself becomes large, thus causing the temperature of the induction coil and the neighborhood thereof to rise. This rise in the temperature demands that some safety measures be taken.